442
M. Fabiano et aI.
compared to other enclosed marine basins or
Mediterranean coastal lagoons, were quite low
indicating that this system, although extremely
rich in organic detritus, receives low inputs of
primary organic matter deriving from micro algae. Moreover, the contribution of primary organic carbon to the sedimentary BPC, calculated
assuming the conversion factor of 40 Jlg C Jlg SChi-a"! (de longe 1980), was quite low (on average, 2.2%), being 30-times lower than values
encountered in the oligotrophic Ligurian Sea
(Fabiano et al. 1995). Although, the C to Chl-a
ratio can vary from about 10 to 100, the contribution of primary organic carbon to the sedimentary biopolymeric fraction of organic matter
(OM) in the present study remains still low (0.6%
and 5.6%, using 10 and 100 as conversion factors,
respectively). Thus, we can conclude that most of
the sedimentary OM in the Marsala lagoon originates from sources different from microphytobenthos or deposited phytoplankton.
In the sediments of the Marsala lagoon, the
three main biochemical classes of organic compounds showed the highest concentrations ever
reported in the literature. The biochemical composition of the sedimentary organic detritus
showed the dominance of carbohydrates (on average, 51.2%) followed by proteins (39.0%) and
lipids (9.8%). The dominance of carbohydrates
over proteins is characteristic of highly oligotrophic or detrital (such as the deep-sea) environments (Danovaro et al. 1993). Organic nitrogen (i.e. protein) is widely considered to be the
major limiting factor for deposit-feeders. Protein
concentrations in the sediments of the Marsala
lagoon were very high, even when compared
with those encountered in extremely highly productive areas such as the coastal sediments of the
Baltic Sea (Meyer-Reil 1983). However. protein
accounted only for less than 40% of the sedimentary biopolymeric carbon indicating that. although large amounts of detrital organic matter were
present during the study period in the sediments
of the Marsala lagoon, this detritus was relatively
of low nutritional quality. This appeared even
more clear when looking at the protein to carbohydrate (PRT:CHO) values. Low values of the
PRT:CHO ratio suggest the presence of aged
organic matter (Danovaro et al.1993). No significant seasonal or spatial changes were found in
PRT:CHO ratio values in the study area.
Moreover, for most of the study period, the ratio
remained lower than 1.0. This result confirms
that the sediments of the Marsala lagoon, for
almost the entire year of investigation, were characterised by the presence of large amounts of
aged andlor non-living organic matter.
Environmental Constraints on Quality
and Quantity of Organic Matter
Temperature and salinity reported for the study
area are similar to those of other typical
Mediterranean coastal environments (Chassany
de Casabianca 1979; Hamon et al. 1979; Zaouali
and Baeten 1983; Friligos 1989; Pusceddu and
Fabiano 1996). Although these variables showed
significant seasonal changes, they have been
reported as having only a secondary role in affecting seston dynamics in the Marsala lagoon.
Indeed, Pusceddu et al. (1997a,b) showed that
the southern area of the study site is characterised by constant sea vivification and by the presence of freshly generated detritus. In contrast,
they found that the northern area is hardly
influenced by the open sea and is characterised
by the presence of particulate organic matter
deriving mostly from wind-wave induced resuspension. The Marsala lagoon is characterised
also by a clear southward positive gradient of
algal coverage (Calvo et al.1996). In the northern
sector, highly turbulent water dynamics affect
significantly the possibility for algae to colonise
these sediments. Here, large amounts of total
suspended matter have been measured and related to the frequent and persistent sediment resuspension. The large amount of mineral seston in
the northern area (annual average about 90% of
TSM concentrations at stations 1-3), and the
large dilution of the biopolymeric fraction of
particulate organic matter (in terms of
POM:TSM ratio, approximately 17%), could be
further indirect signals of sediment resuspension. Accordingly. the lowest amounts of sediment organic matter (in terms of S-BPC), characterised by the co-dominance of proteins and
carbohydrates, were found in the northern area
(station A).
Moving southward and approaching the central area, Cymodocea nodosa becomes dominant, gradually substituted by a Posidonia oceanica reef. In this area, concentrations of total
suspended matter and of all its biochemical
components, with the exception of particulate
proteins. gradually decrease, as a result of the
M. Fabiano et aI.
compared to other enclosed marine basins or
Mediterranean coastal lagoons, were quite low
indicating that this system, although extremely
rich in organic detritus, receives low inputs of
primary organic matter deriving from micro algae. Moreover, the contribution of primary organic carbon to the sedimentary BPC, calculated
assuming the conversion factor of 40 Jlg C Jlg SChi-a"! (de longe 1980), was quite low (on average, 2.2%), being 30-times lower than values
encountered in the oligotrophic Ligurian Sea
(Fabiano et al. 1995). Although, the C to Chl-a
ratio can vary from about 10 to 100, the contribution of primary organic carbon to the sedimentary biopolymeric fraction of organic matter
(OM) in the present study remains still low (0.6%
and 5.6%, using 10 and 100 as conversion factors,
respectively). Thus, we can conclude that most of
the sedimentary OM in the Marsala lagoon originates from sources different from microphytobenthos or deposited phytoplankton.
In the sediments of the Marsala lagoon, the
three main biochemical classes of organic compounds showed the highest concentrations ever
reported in the literature. The biochemical composition of the sedimentary organic detritus
showed the dominance of carbohydrates (on average, 51.2%) followed by proteins (39.0%) and
lipids (9.8%). The dominance of carbohydrates
over proteins is characteristic of highly oligotrophic or detrital (such as the deep-sea) environments (Danovaro et al. 1993). Organic nitrogen (i.e. protein) is widely considered to be the
major limiting factor for deposit-feeders. Protein
concentrations in the sediments of the Marsala
lagoon were very high, even when compared
with those encountered in extremely highly productive areas such as the coastal sediments of the
Baltic Sea (Meyer-Reil 1983). However. protein
accounted only for less than 40% of the sedimentary biopolymeric carbon indicating that. although large amounts of detrital organic matter were
present during the study period in the sediments
of the Marsala lagoon, this detritus was relatively
of low nutritional quality. This appeared even
more clear when looking at the protein to carbohydrate (PRT:CHO) values. Low values of the
PRT:CHO ratio suggest the presence of aged
organic matter (Danovaro et al.1993). No significant seasonal or spatial changes were found in
PRT:CHO ratio values in the study area.
Moreover, for most of the study period, the ratio
remained lower than 1.0. This result confirms
that the sediments of the Marsala lagoon, for
almost the entire year of investigation, were characterised by the presence of large amounts of
aged andlor non-living organic matter.
Environmental Constraints on Quality
and Quantity of Organic Matter
Temperature and salinity reported for the study
area are similar to those of other typical
Mediterranean coastal environments (Chassany
de Casabianca 1979; Hamon et al. 1979; Zaouali
and Baeten 1983; Friligos 1989; Pusceddu and
Fabiano 1996). Although these variables showed
significant seasonal changes, they have been
reported as having only a secondary role in affecting seston dynamics in the Marsala lagoon.
Indeed, Pusceddu et al. (1997a,b) showed that
the southern area of the study site is characterised by constant sea vivification and by the presence of freshly generated detritus. In contrast,
they found that the northern area is hardly
influenced by the open sea and is characterised
by the presence of particulate organic matter
deriving mostly from wind-wave induced resuspension. The Marsala lagoon is characterised
also by a clear southward positive gradient of
algal coverage (Calvo et al.1996). In the northern
sector, highly turbulent water dynamics affect
significantly the possibility for algae to colonise
these sediments. Here, large amounts of total
suspended matter have been measured and related to the frequent and persistent sediment resuspension. The large amount of mineral seston in
the northern area (annual average about 90% of
TSM concentrations at stations 1-3), and the
large dilution of the biopolymeric fraction of
particulate organic matter (in terms of
POM:TSM ratio, approximately 17%), could be
further indirect signals of sediment resuspension. Accordingly. the lowest amounts of sediment organic matter (in terms of S-BPC), characterised by the co-dominance of proteins and
carbohydrates, were found in the northern area
(station A).
Moving southward and approaching the central area, Cymodocea nodosa becomes dominant, gradually substituted by a Posidonia oceanica reef. In this area, concentrations of total
suspended matter and of all its biochemical
components, with the exception of particulate
proteins. gradually decrease, as a result of the